Electrical assembly having impedance controlled signal traces

ABSTRACT

An electrical assembly having controlled impedance signal traces and a portable electronic device comprising an electrical assembly having controlled impedance signal traces are provided. In accordance with one embodiment, there is provided a portable electronic device, comprising an electrical assembly, comprising: a chassis made from a conductive material and forming a first ground plane; a first dielectric substrate layer overlaying the chassis; a first signal trace overlaying the first dielectric substrate layer; and a second dielectric layer overlaying the first signal trace.

RELATED APPLICATION DATA

The present application is a divisional application of U.S. patentapplication Ser. No. 12/609,144, filed Oct. 30, 2009, the content ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to the control of electrical impedances,and in particular to portable electronic devices comprising anelectrical assembly having impedance controlled signal traces.

BACKGROUND

Electronic devices, including portable electronic devices, have gainedwidespread use and may provide a variety of functions including, forexample, telephonic, electronic messaging and other personal informationmanager (PIM) application functions. Portable electronic devicesinclude, for example, several types of mobile stations such as simplecellular telephones, smart telephones, wireless personal digitalassistants (PDAs), and laptop computers with wireless 802.11 orBluetooth™ capabilities.

Portable electronic devices such as PDAs or smart telephones aregenerally intended for handheld use and ease of portability. Smaller andthinner portable electronic devices are generally desirable forportability. The number, type, location and mounting of the electroniccomponents of portable electronic devices affect the size and profile(e.g., thickness) of portable electronic devices. Accordingly, devicedesigns, configurations and features which provide smaller and thinnerportable electronic devices are desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of components including internalcomponents of a portable electronic device in accordance with oneexample embodiment;

FIG. 2 is a front view of an example embodiment of a portable electronicdevice in a portrait orientation;

FIG. 3 is a sectional side view of portions of the portable electronicdevice of FIG. 2;

FIG. 4A is a front view of a chassis of the portable electronic deviceof FIG. 1 in accordance with one example embodiment;

FIG. 4B is a sectional side view of the chassis of FIG. 4A taken alongthe line 4B-4B and in the direction indicated;

FIG. 5A is a front view of the chassis of FIG. 4A with a flexibleprinted circuit board mounted with a recess of the chassis;

FIG. 5B is a sectional side view of the chassis of FIG. 5A taken alongthe line 5B-5B and in the direction indicated;

FIG. 6 is a front view of the chassis of FIG. 5A with a main circuitboard attached to the flexible printed circuit board;

FIGS. 7A to 7C are sectional views of an electrical assembly inaccordance with example embodiments of the present disclosure;

FIGS. 8A to 8C are sectional views of a flexible printed circuit boardin accordance with example embodiments of the present disclosure;

FIG. 9A is a schematic diagram of a microstrip circuit structure;

FIG. 9B is a schematic diagram of an embedded microstrip circuitstructure; and

FIG. 9C is a schematic diagram of a stripline circuit structure.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

For simplicity and clarity of illustration, reference numerals may berepeated among the figures to indicate corresponding or analogouselements. Numerous details are set forth to provide an understanding ofthe embodiments described herein. The embodiments may be practicedwithout these details. In other instances, well-known methods,procedures, and components have not been described in detail to avoidobscuring the embodiments described. The description is not to beconsidered as limited to the scope of the embodiments described herein.

In accordance with one embodiment of the present disclosure, there isprovided an electrical assembly, comprising: a chassis for mountingelectronic components, the chassis being made from a conductive materialand forming a first ground plane; a first dielectric layer overlayingthe chassis; a first signal trace overlaying the first dielectric layer;and a second dielectric layer overlaying the first signal trace.

In accordance with another embodiment of the present disclosure, thereis provided a portable electronic device, comprising: an electricalassembly, comprising: a chassis made from a conductive material andforming a first ground plane; a dielectric substrate layer overlayingthe chassis; and a first signal trace embedded within the dielectricsubstrate layer; a main circuit board mounted to the chassis andincluding multiple connectors for connecting electronic components, themain circuit board being connected to the first signal trace; aprocessor connected to the main circuit board; at least one input deviceconnected to the main circuit board and to the processor through themain circuit board; and at least one output device connected to the maincircuit board and to the processor through the main circuit board.

The disclosure generally relates to an electronic device, which is aportable electronic device in the embodiments described herein. Examplesof portable electronic devices include mobile, or handheld, wirelesscommunication devices such as pagers, cellular phones, cellularsmart-phones, wireless organizers, personal digital assistants,wirelessly enabled notebook computers, and so forth. The portableelectronic device may also be a portable electronic device withoutwireless communication capabilities, such as a handheld electronic gamedevice, digital photograph album, digital camera, or other device.

A block diagram of an example of a portable electronic device 100 isshown in FIG. 1. The portable electronic device 100 includes multiplecomponents, such as a processor 102 that controls the overall operationof the portable electronic device 100. Communication functions,including data and voice communications, are performed through acommunication subsystem 104. Data received by the portable electronicdevice 100 is decompressed and decrypted by a decoder 106. Thecommunication subsystem 104 receives messages from and sends messages toa wireless network 150. The wireless network 150 may be any type ofwireless network, including, but not limited to, data wireless networks,voice wireless networks, and networks that support both voice and datacommunications. A power source 142, such as one or more rechargeablebatteries or a port to an external power supply, powers the portableelectronic device 100.

The processor 102 interacts with other components, such as Random AccessMemory (RAM) 108, memory 110, a display screen 112 (such as a liquidcrystal display (LCD)) with a touch-sensitive overlay 114 operablyconnected to an electronic controller 116 that together comprise atouch-sensitive display 118, a digital camera 120, one or more auxiliaryinput/output (I/O) subsystems 124, a data port 126, a speaker 128, amicrophone 130, short-range communications subsystem 132, and otherdevice subsystems 134. User-interaction with a graphical user interface(GUI) is performed through the touch-sensitive overlay 114. Theprocessor 102 interacts with the touch-sensitive overlay 114 via theelectronic controller 116. Information, such as text, characters,symbols, images, icons, and other items that may be displayed orrendered on the portable electronic device, is displayed on thetouch-sensitive display 118 via the processor 102.

The auxiliary I/O subsystems 124 may include one or more of thefollowing input devices: one or more control keys, a keyboard, a keypad,or a navigation mechanism. The navigation mechanism may be aclickable/depressible trackball, a clickable/depressible scroll wheel, atouch-sensitive optical trackpad, or a touch-sensitive touchpad. Theauxiliary I/O subsystems 124 may include one or more of the outputdevices to supplement the touch-sensitive display 118 and speaker 128including, but not limited to, a notification light such as a lightemitting diode (LED) or a vibrator or other mechanism for providinghaptic/touch feedback.

To identify a subscriber for network access, the portable electronicdevice 100 uses a Subscriber Identity Module or a Removable UserIdentity Module (SIM/RUIM) card 138 for communication with a network,such as the wireless network 150. Alternatively, user identificationinformation may be programmed into memory 110.

The portable electronic device 100 includes an operating system 146 andsoftware applications or programs 148 that are executed by the processor102 and are typically stored in a persistent, updatable store such asthe memory 110. Additional applications or programs 148 may be loadedonto the portable electronic device 100 through the wireless network150, the auxiliary I/O subsystem 124, the data port 126, the short-rangecommunications subsystem 132, or any other suitable subsystem 134.

A received signal such as a text message, an e-mail message, or web pagedownload is processed by the communication subsystem 104 and input tothe processor 102. The processor 102 processes the received signal foroutput to the display screen 112 and/or to the auxiliary I/O subsystem124. A subscriber may generate data items, for example e-mail messages,which may be transmitted over the wireless network 150 through thecommunication subsystem 104. For voice communications, the overalloperation of the portable electronic device 100 is similar. The speaker128 outputs audible information converted from electrical signals, andthe microphone 130 converts audible information into electrical signalsfor processing.

FIG. 2 shows a front view of an example of the portable electronicdevice 100 in portrait orientation. The portable electronic device 100includes a housing 200 that houses internal components includinginternal components shown in FIG. 1 and frames the touch-sensitivedisplay 118 such that the touch-sensitive display 118 is exposed foruser-interaction therewith when the portable electronic device 100 is inuse. It will be appreciated that the touch-sensitive display 118 mayinclude any suitable number of user-selectable features renderedthereon, for example, in the form of virtual buttons for user-selectionof, for example, applications, options, or keys of a keyboard for userentry of data during operation of the portable electronic device 100.

The touch-sensitive display 118 provides both an input device and outputdevice for the portable electronic device 100. The touch-sensitivedisplay 118 may be any suitable touch-sensitive display, such as acapacitive, resistive, infrared, surface acoustic wave (SAW)touch-sensitive display, strain gauge, optical imaging, dispersivesignal technology, acoustic pulse recognition, and so forth, as known inthe art. A capacitive touch-sensitive display includes the capacitivetouch-sensitive overlay 114. The overlay 114 may be an assembly ofmultiple layers in a stack including, for example, a substrate, a groundshield layer, a barrier layer, one or more capacitive touch sensorlayers separated by a substrate or other barrier, and a cover. Thecapacitive touch sensor layers may be any suitable material, such aspatterned indium tin oxide (ITO).

One or more touches, also known as touch contacts or touch events, maybe detected by the touch-sensitive display 118. The processor 102 maydetermine attributes of the touch, including a location of a touch.Touch location data may include an area of contact or a single point ofcontact, such as a point at or near a centre of the area of contact. Thelocation of a detected touch may include x and y components, e.g.,horizontal and vertical components, respectively, with respect to one'sview of the touch-sensitive display 118. For example, the x locationcomponent may be determined by a signal generated from one touch sensor,and the y location component may be determined by a signal generatedfrom another touch sensor. A signal is provided to the controller 116 inresponse to detection of a touch. A touch may be detected from anysuitable object, such as a finger, thumb, appendage, or other items, forexample, a stylus, pen, or other pointer, depending on the nature of thetouch-sensitive display 118. Multiple simultaneous touches may bedetected.

The housing 200 can be any suitable housing for the internal componentsshown in FIG. 1. FIG. 3 shows a sectional side view of portions of theportable electronic device 100. The housing 200 in the present exampleincludes a back 202, a frame 204, which frames the touch-sensitivedisplay 118 and sidewalls 206 that extend between and generallyperpendicular to the back 202 and the frame 204. In the shownembodiment, the portable electronic device 100 includes a base 208 whichis spaced apart from and is generally parallel to the back 202. The base208 can be any suitable base. In at least some embodiments, the base 208may include a circuit interconnect structure which could be, in someembodiments, a main circuit board 460 (FIG. 6) of the portableelectronic device 100. The main circuit board 460 is typically a rigidprinted circuit board (PCB), but may be a flexible PCB supported by astiff support between the base 208 and the back 202, or a hybrid circuitstructure comprising a flexible and rigid PCB. The back 202 may includea plate (not shown) that is releasably attached for insertion andremoval of, for example, the power source 142 and the SIM/RUIM card 138referred to above. It will be appreciated that the back 202, thesidewalls 206 and the frame 204 may be injection molded, for example. Inthe example of the portable electronic device 100 shown in FIG. 3, theframe 204 is generally rectangular with rounded corners, although othershapes are possible.

The display screen 112 and the touch-sensitive overlay 114 are supportedon a chassis 210 (also known as a support frame or tray) for providingmechanical support to the display screen 112 and touch-sensitive overlay114. The chassis 210 defines a cavity between its sidewalls forreceiving electronics components such as the display screen 112 andtouch-sensitive overlay 114. The received electronics are mounteddirectly or indirectly to the chassis 210. The chassis 210 is made froma suitable electrically conductive material which provides themechanical support which forms a first ground plane for the portableelectronic device 100. The first ground plane is used in controllingelectrical impedances of one or more transmission lines of the portableelectronic device 100, as described in more detail below. In someembodiments, the chassis 210 is made from a magnesium alloy, forexample, via casting. In other embodiments, the chassis 210 is made froma suitable electrically conductive stainless steel alloy.

Referring now to FIG. 4A to 6, one example embodiment of the chassis 210will be described. FIG. 4A is a front view of the chassis 210 and FIG.4B is a sectional side view of the chassis 210. The chassis 210 has afront surface 401 in which a recess 402 is defined. In the shownembodiment, the recess 402 is generally elongate and defines a pathextending between a top portion of the chassis 210 and a bottom portionof the chassis 210. However, the recess 402 may define a different pathand shape in other embodiments.

In the shown embodiment, three circuit boards 410, 420 and 430 aremounted to the front surface 401 of the chassis 210 in its top portion.A different number of circuit boards may be provided in otherembodiments. Moreover, the circuits could be mounted elsewhere in otherembodiments, for example, on the bottom portion of the chassis 210 or ona side of the chassis 210. In some embodiments, the circuit boards 410,420 and 430 are rigid printed circuit boards but could be any othersuitable type of circuit interconnect structure. Each of the circuitboards 410, 420 and 430 has an electronic component mounted to it asindicated by references 412, 422, and 432 respectively. At least some ofthe electronic components 412, 422, and 432 may be relatively high speedcomponents requiring relative high data transmission rates forexchanging data with the processor 102 of the the portable electronicdevice 100. Examples of high speed components which may comprise one ormore of the electronic components 412, 422, and 432 are the digitalcamera 120, the touch-sensitive display 118 or an LCD screen (notshown), memory buses (not shown), or other electronic component havinghigh speed signals with impedance matching requirements. One or more ofthe electronic components 412, 422, and 432 may not be high speedcomponents, examples of non-high speed electronic components which maycomprise one or more of the electronic components 412, 422, and 432 arean audio jack or port (not shown) for outputting audio data andbypassing the speaker 128 and the data port 126.

In the shown embodiment, supports 440, 442 are attached to the chassis210 for supporting electronic components of the the portable electronicdevice 100 which are mounted directly or indirectly to chassis 210. Inthe shown embodiment, at least one first support 440 is attached to thechassis 210 for supporting a connector or interface for connecting dataand/or power transmission lines connecting the electronic components412, 422, and 432 to the main circuit board 460 (FIG. 6) of the portableelectronic device 100. It will be understood that the electroniccomponents 412, 422, and 432 are connected to the processor 120 of theportable electronic device 100 through the main circuit board 460. Inthe shown embodiment, the main circuit board 460 is not attached to thebase 208 nor does it form part of the base 208 as in FIG. 2. In theshown embodiment of FIGS. 4A to 6, the base 208 may be omitted.

A plurality of second supports 442 are attached to the chassis 210 forsupporting the main circuit board 460 (FIG. 6). The first support 440may also indirectly support the main circuit board 460. In the shownembodiment, the first support 440 and second supports 442 are locatedwithin the recess 402 at the bottom portion of the chassis 210; however,the first support 440 and second supports 442 could be attached to thefront surface 401 in other embodiments depending, for example, on theconfiguration of the recess 402. In at least some embodiments, the firstsupport 440 and second supports 442 are foam supports.

Referring to FIGS. 5A and 5B, a flexible printed circuit board (PCB) 450which carries data and power transmission lines connecting theelectronic components 412, 422, and 432 to the main circuit board 460 ofthe portable electronic device 100 is shown. In other embodiments, theflexible PCB 450 could be replaced with a rigid PCB. In otherembodiments, the flexible PCB 450 may carry only data transmission linesor power transmission lines. The flexible PCB 450 is connected to thecircuit boards 410, 420 and 430, typically using an interface connector452 for connecting with a corresponding interface (not shown) on themain circuit board 460. The interface connector 452 is a suitable datainterface, power interface, or data and power interface depending on thetype of transmission line carried by the flexible PCB 450. FIG. 6 showsthe chassis 210 with the main circuit board 460 connected to theinterface connector 452 of the flexible PCB 450. Alternatively, theflexible PCB 450 could be connected to the circuit boards 410, 420 and430 without an interface connector 452 using a conductive adhesive filmor similar techniques for making a suitable electrical connection.

The flexible PCB 450 is received in the recess 402 in the chassis 210 asshown in FIGS. 5A and 5B. The recess 402 is typically sized so that atop surface of the flexible PCB 450 is flush with the front surface 401of the chassis 210 or is located just below the front surface 401. Therecess 402 is at least as deep as the flexible PCB 450 is thick. When aconductive adhesive or conductive film is used to electrically connectthe flexible PCB 450 to the chassis 210, the depth of recess 402 shouldaccount for the thickness of the flexible PCB 450 and the conductiveadhesive or conductive film. For example, in some embodiments theflexible PCB 450 is 0.2 to 0.4 mm in thickness and a 0.05 mm conductiveadhesive or film is used. Accordingly, the recess 402 should be at least0.25 to 0.45 mm in depth to allow for the flexible PCB 450 andconductive adhesive or film so that the top surface of the flexible PCB450 is flush with the front surface 401 of the chassis 210 or is locatedjust below it. The recess 402 reduces the overall thickness of theportable electronic device 100 and increases the volume of spaceavailable for electronics components to be received in the cavitydefined by the chassis 210 and mounted thereto.

The recess 402 is at least the width of the flexible PCB 450 plus apredetermined mechanical tolerance; however, it may be wider in someareas. In the bottom portion of the chassis 210 where the first support440 and second supports 442 are located, the recess 402 changes in shapefrom a track or channel which generally matches the shape of theflexible PCB 450 to an enlarged area which occupies substantially all ofthe bottom portion of the chassis 210. The main PCB 460 is mounted inthis enlarged area in at least some embodiments. In other embodiments,the recess 402 may be omitted and the flexible PCB 450 attached directlyto the front surface 401 of the chassis 210.

The flexible PCB 450 is typically mounted or attached to a bottom of therecess 402 using an electrically conductive adhesive. The electricallyconductive adhesive may be an electrically conductive glue or pasteapplied to the bottom of recess 402 at some or all of the locationswhich are located adjacent to the flexible PCB 450, an electricallyconductive pressure-sensitive adhesive (PSA) attached to the bottom ofrecess 402 at some or all of the locations which are located adjacent tothe flexible PCB 450, or any other suitable electrically conductiveadhesive. In other embodiments, a non-conductive adhesive may be usedinstead of an electrically conductive adhesive. In other embodiments,the flexible PCB 450 may secured to the chassis 210 using mechanical orother means and the adhesive may be omitted. The PSA, whether conductiveor non-conductive, is typically 0.03 mm to 0.07 mm in thickness, andmore typically 0.05 mm in thickness as noted above. The thickness rangesbased between embodiments based on the particular design considerationsand the particular PSA being used.

The above-described embodiments provide an electrical assembly having arigid-flex printed circuit board in which the flexible PCB 450 connectsto a rigid main printed circuit board 460 and rigid printed circuitboards 410, 420 and 430 through the flexible PCB 450. The rigid-flexprinted circuit board is then attached to the conductive chassis 210which acts as a ground plane for at least one transmission line ofrigid-flex printed circuit board such as, for example, a transmissionline of the flexible PCB 450. However, the teachings of the presentdisclosure can be applied to a flexible printed circuit board on itsown, a rigid printed circuit board on its own, or discrete circuitinterconnect structure requiring a ground plane. Moreover, as notedabove, the flexible PCB 450 could be replaced with a rigid PCB. Whileoperative, an electrical assembly created by the chassis 210, the mainPCB 460 and a rigid PCB providing the connecting functions of theflexible PCB 450 may be thicker relative to described embodiment inwhich a flexible PCB 450 is used.

Referring now to FIGS. 7A to 7C, example embodiments of an electricalassembly in accordance with the present disclosure will be described.FIG. 7A shows a first embodiment of an electrical assembly. Theelectrical assembly comprises the grounded chassis 210 of the portableelectronic device 100. As described above, the chassis 210 is made froma conductive material such as a conductive magnesium alloy which, forexample, may be formed by casting. The conductive chassis 210 forms afirst ground plane for the device. The chassis 210 is also used formounting various electronic components such as the printed circuitboards 410, 420 and 430.

The electrical assembly further comprises a first dielectric layer 706,i.e., a non-conducting electrically insulating layer, overlaying thechassis 210. The first dielectric layer 706 is a dielectric substratewhich may be made from suitable dielectric polymer such as, for example,a polyimide polymer. A first signal trace 712 overlays the firstdielectric layer 706. A second dielectric layer 710 overlays the firstsignal trace 712. If no signal lines are located above the signal trace,the second dielectric layer 710 may be air rather than a dielectricsubstrate as in the shown embodiment. In other embodiments, the seconddielectric layer 710 is dielectric substrate which may be made from asuitable dielectric polymer such as, for example, a polyimide polymer.

When the second dielectric layer 710 is a dielectric substrate, it maybe the same or different from the dielectric substrate of the firstdielectric layer 706. In at least some embodiments, the first dielectriclayer 706, first signal trace 712 and second dielectric layer 710 areintegrally formed as part of a circuit interconnect structure such as aflexible printed circuit board 450 (FIGS. 5A to 6). In such embodiments,the first dielectric layer 706 of the electrical assembly forms thebottom layer of the flexible PCB 450 and is attached to the chassis 210typically within the recess 402 using a conductive adhesive 720. Inother embodiments, the first dielectric layer 706 may be attached to thefront surface 401 rather than within a recess 402, may be attached usinga non-conductive adhesive, or may be secured to the chassis 210 usingmechanical or other means and the adhesive 720 may be omitted. As notedabove, the conductive layer may be an electrically conductive PSA, glueor paste.

When mounted to the conductive chassis 210, the electrical assembly ofFIG. 7A creates a microstrip circuit in which the conductive chassis 210acts a ground plane for the first signal trace of the microstripcircuit. This eliminates the lower ground layer that would be requiredusing a conventional flexible PCB and eliminates the mechanical stack-upwhich would have been required had the first signal trace been aninternal circuit layer of the flexible PCB 450.

FIG. 7B shows a second embodiment similar to that shown in FIG. 7A withthe notable difference that the second dielectric layer 710 iscombined/integrally formed with the first dielectric layer 706 to form acombined dielectric substrate layer 708 in which the first signal trace712 is embedded. The combined dielectric substrate layer 708 not onlyoverlays the first signal trace 712 but surrounds it. When mounted tothe conductive chassis 210, the electrical assembly of FIG. 7B createsan embedded microstrip circuit in which the conductive chassis 210 actsa ground plane for the first signal trace of the embedded microstripcircuit.

FIG. 7C shows a third embodiment similar to that shown in FIG. 7B withthe notable difference that a second ground plane 716 overlays thecombined dielectric substrate 708. When mounted to the conductivechassis 210, the electrical assembly of FIG. 7C creates a striplinecircuit in which the conductive chassis 210 acts a first ground planefor the first signal trace of the stripline circuit. Conductive vias(not shown) extend through the stripline circuit to connect the secondground plane to the chassis 210 and the first ground plane formed by it.

The present disclosure describes an electrical assembly which uses agrounded chassis as a ground layer for a circuit interconnect structuresuch as a flexible PCB. This allows one or more solid ground layers inthe circuit interconnect structure to be eliminated depending on thetype and configuration of the signal traces in the stack-up of thecircuit interconnect structure. This allows the overall thickness of thecircuit interconnect structure (e.g., flexible PCB) to be reduced whilemaintaining the required electrical parameters and ground reference.

Referring now to FIGS. 8A to 8C, example embodiments of the flexible PCB450 will be described. The flexible PCB 450 comprises one or morecircuit layers, typically multiple circuit layers, each circuit layerbeing made up of several sub-layers and comprising one or more traces.In a typical flexible PCB 450 with multiple circuit layers, the firstsignal trace 712 of the electrical assembly of FIG. 7B or 7C is locatedin the bottom circuit layer. The additional circuit layers overlay thecircuit layer having the first signal trace 712 using a mechanicalstack-up configured for the multiple circuit layers of the flexible PCB450. Each circuit layer in the flexible PCB 450 is typically separatedby a spacer formed from a dielectric material. The flexible PCB 450 alsohas a protective covering or sheath (not shown) as is known in the art.

Examples of single circuit structures for a circuit layer which may beincluded in the flexible PCB 450 are microstrip, embedded microstrip,symmetrical stripline or asymmetrical stripline. Microstrip signaltraces are useful in high-speed digital PCB designs where signals needto be routed from one part of the assembly to another with minimaldistortion while reducing or minimizing high cross-talk between signaltraces. Generally, stripline circuit structures achieve isolationbetween adjacent signal traces more easily than microstrip circuitstructures. However, stripline circuit structures are generally harderand more expensive to fabricate than microstrip circuit structures.

For multi-circuit flexible PCBs, the outer layers of the flexible PCB450 are typically microstrip circuit layers (referred to collectively as“M” layers) and the inner layers of the flexible PCB 450 are typicallysymmetrical stripline or asymmetrical stripline circuit layers (referredto collectively as “S” layers) sandwiched between the microstrip circuitlayers. A multi-circuit stack-up requires the bottom microstrip circuitlayer to be an embedded microstrip trace; however, the top layermicrostrip circuit layer need not be an embedded microstrip trace. Thus,a typical multi-circuit stack-up for the flexible PCB 450 has a generaltop-to-bottom configuration of M-(S)_(n)-M where n is the number ofstripline circuit layers in the middle of the flexible PCB 450. Thenumber of stripline circuit layers is at least one, but a typical rangeis 3 to 7 stripline circuit layers, often 5 or 6 stripline circuitlayers. As noted above, electronic component having high speed signalswith impedance matching requirements such as the digital camera 120 areconnected to one of the microstrip circuit layers which, in at leastsome embodiments, is the bottom layer which is electrically connected tothe chassis 210. In some embodiments, the signal trace for more than onehigh speed component (e.g., the digital camera 120 and thetouch-sensitive display 118 or LCD screen) may be located in the samecircuit layer, for example, in a coupled embedded microstrip layer whichmakes up the bottom layer of the flexible PCB 450.

It is possible that the flexible PCB 450 could be configured to includeother single circuit structures such as, for example, a surface coplanarwaveguide trace and ground plane or embedded coplanar waveguide traceand ground plane in the flexible PCB 450. It is also possible that theflexible PCB 450 could be configured to include multiple circuitstructure for a circuit layer such as, for example, edge-coupledmicrostrip, edge-coupled embedded microstrip, edge-coupled symmetricalstripline, edge-coupled asymmetrical stripline, broadside-coupledstripline and offset broadside-coupled stripline circuit structures. Theconstruction of the above-mentioned circuit structures is well known inthe art and will not be described in detail herein.

Each of the above-mentioned circuits includes at least one transmissionline also known as a signal line. Each signal line is formed by a signaltrace which comprises a conductive foil which has been appropriatelypatterned onto a dielectric substrate into a desired circuit patternusing, for example, conventional photolithography (or masking) andetching techniques. The conductive foil that forms the signal trace isthe conductive material which remains after etching. The conductive foilis typically copper but could be another suitable conductive material.The flexible PCB 450 typically also comprises power and ground traces.The signal traces are usually narrower than power or ground tracesbecause the current carrying requirements of signal traces are usuallymuch less.

Each signal trace has a characteristic electrical impedance, referred toherein as simply impedance, which measures the opposition to asinusoidal alternating current (AC) experienced by the signal trace. Theimpedance of the signal trace is affected by the circuit topology, thedielectric constant of the dielectric layer(s) surrounding it, thedielectric height of the respective circuit layer, the width of thesignal trace, and the thickness of the signal trace. Proper control ofthe impedance of a signal trace may reduce noise and maintain signalintegrity within predetermined tolerances, among other benefits.

FIG. 8A shows a first embodiment of the flexible PCB 450 in which thebottom circuit layer is an embedded microstrip signal trace as shown inFIG. 7B. Overlaying the bottom circuit layer are intermediate circuitlayers 802 separated by spacers 804 formed for a suitable dielectricmaterial. The top layer of the flexible PCB 450 in the embodiment ofFIG. 8A is a microstrip comprising a second ground plane 812 whichoverlays the uppermost spacer 804, a third dielectric substrate layer814 which overlays the second ground plane 812, and a second signaltrace 816 which overlays the third dielectric substrate layer 814. Theair surrounding the signal trace 816 acts as a fourth dielectric layer818 as in microstrip signal trace of FIG. 7A. Multiple conductive vias820 extend through the flexible PCB 450 and connect the ground planes ofall of the circuit layers 802 to the conductive chassis 210. Theconductive vias 820 are formed by holes through the flexible PCB 450which are made conductive by electroplating and having a pad or contacton their exposed side of the flexible PCB 450 for contacting the chassis210 and connecting the ground planes to the chassis 210. Alternatively,edge contacts of the flexible PCB 450 could be used for connecting theground planes to the chassis 210.

FIG. 8B shows a second embodiment of the flexible PCB 450 similar tothat shown in FIG. 8A with the notable difference that the top circuitlayer is an embedded microstrip signal trace rather than an ordinarymicrostrip signal trace as in FIG. 8A. In the embodiment of FIG. 8B, thethird dielectric layer 814 and fourth dielectric layer 818 of the firstembodiment of FIG. 8A are combined/integrally formed to form a combineddielectric substrate layer 822 in which the second signal trace 816 isembedded. The combined dielectric substrate layer 822 not only overlaysthe second signal trace 816 but surrounds it.

FIG. 8C shows a third embodiment of the flexible PCB 450 similar to thatshown in FIG. 8B with the notable difference that the top circuit layeris a stripline signal trace rather than an embedded microstrip signaltrace as in FIG. 8B. In the embodiment of FIG. 8C, a ground layer 824overlays the dielectric layer 822 and is connected to the conductivevias 820 connecting it to the chassis 210.

Referring now to FIGS. 9A to 9C, the calculation of impedance formicrostrip, embedded microstrip and stripline circuit structures will bebriefly described. The equations mentioned below are derived by theInstitute for Interconnecting and Packaging Electronic Circuits (IPC)and are further described in the Design Guide for Electronic PackagingUtilizing High-Speed Techniques (4^(th) Working Draft, IPC-2251,February 2001) (hereinafter the “Design Guide”), which is incorporatedherein by reference. Other equations may be used for calculation ofimpedance for microstrip, embedded microstrip and stripline circuitstructures, or other circuit structures.

Due to the effect of the environment on the impedance of a signal trace,impedance can typically only be approximated based on the designequations of the Design Guide. When designing a printed circuit boardfor a particular application, an iterative process may be required inwhich a printed circuit is designed based on assumptions about itsoperating environment and a first prototype is prepared and testedin-situ to measure the characteristic impedance of its signal traces.The design is then modified to take into account environmental effectswhen the measured impedance is outside of predetermined tolerances. Afurther prototype is then prepared based on the modified design which isthen tested in-situ, and the design is further modified if necessary.This process is repeated until the design is within predeterminedtolerances.

FIG. 9A shows a schematic diagram of a microstrip circuit structure. Anapproximation of the impedance of a microstrip signal trace may becalculated in accordance with the following equation:

$Z_{0} = {\frac{87}{\sqrt{e_{r} + 1.41}}{{Ln}\left\lbrack \frac{5.98H}{\left( {{{.8}W} + T} \right)} \right\rbrack}}$where W is the signal trace width in millimeters (mm), T is signal tracethickness in mm, H is the height of signal trace in mm, ∈_(r) is therelative permittivity of the dielectric (dimensionless) and Z_(o) is thecharacteristic impedance in ohms (Ω). The equation for calculating theimpedance of an embedded microstrip signal trace provided above istypically valid for parameters specified in the Design Guide:0.1<W/H<3.0; 1<∈_(r)<15.

FIG. 9B shows a schematic diagram of an embedded microstrip signaltrace. An approximation of the impedance of an embedded microstripsignal trace may be calculated in accordance with the followingequation:

$Z_{0} = {\frac{87}{\sqrt{{ɛ_{Y} + 1.41}\;}}{\ln\left( \frac{5.98H}{{0.8W} + T} \right)} \times \left( {1 - \frac{H_{1} - T - H}{0.1}} \right)}$where W is the signal trace width in mm, T is signal trace thickness inmm, H is the height of signal trace in mm, H₁ is the height of thedielectric above the ground plane, ∈_(r) is the relative permittivity ofthe dielectric (dimensionless) and Z_(o) is the characteristic impedancein Ω. The equation for calculating the impedance of an embeddedmicrostrip signal trace provided above is typically valid for parametersspecified in the Design Guide: 0.1<W/H<3.0; 1<∈_(r)<15.

FIG. 9C is a schematic diagram of a symmetrical stripline signal trace.An approximation of the impedance of an embedded microstrip signal tracemay be calculated in accordance with the following equation:

$Z_{0} = {\frac{60}{\sqrt{ɛ_{r}}} \times {\ln\left( \frac{4 \times \left( {{2H} + T} \right)}{0.67\pi*\left( {{0.8W} + T} \right)} \right)}}$where W is the signal trace width in mm, T is signal trace thickness inmm, H is the height of signal trace in mm, ∈_(r) is the relativepermittivity of the dielectric (dimensionless) and Z_(o) is thecharacteristic impedance in Ω. The equation for calculating theimpedance of a symmetrical stripline signal trace provided above istypically valid for parameters specified in the Design Guide:W/(H−T)<0.35; T/H<0.25; 1<∈_(r)<15.

While the present disclosure describes example embodiments having atouch-sensitive display 118, it will be appreciated that the teachingsof the present disclosure also apply to portable electronic deviceswhich do not have a touch-sensitive display. Examples of other portableelectronic devices in which the teachings of the present disclosure maybe applied include those having a conventional, non-touch-sensitivedisplay for a display (output) device (e.g., such as an LCD screen) anda keyboard or keypad as an input device. Moreover, the teachings of thepresent disclosure may be applied to portable and non-portableelectronic devices having only an output device or only an input device.

The various embodiments presented above are merely examples and are inno way meant to limit the scope of this disclosure. Variations of theinnovations described herein will be apparent to persons of ordinaryskill in the art, such variations being within the intended scope of thepresent disclosure. In particular, features from one or more of theabove-described embodiments may be selected to create alternativeembodiments comprised of a sub-combination of features which may not beexplicitly described above. In addition, features from one or more ofthe above-described embodiments may be selected and combined to createalternative embodiments comprised of a combination of features which maynot be explicitly described above. Features suitable for suchcombinations and sub-combinations would be readily apparent to personsskilled in the art upon review of the present disclosure as a whole. Thesubject matter described herein and in the recited claims intends tocover and embrace all suitable changes in technology.

The invention claimed is:
 1. A portable electronic device, comprising:an electrical assembly, comprising: a chassis made from a conductivematerial and forming a first ground plane; a first dielectric substratelayer overlaying the chassis; a first signal trace overlaying the firstdielectric substrate layer; and a second dielectric layer overlaying thefirst signal trace; a main circuit board mounted to the chassis andincluding multiple connectors for connecting electronic components, themain circuit board being connected to the first signal trace; aprocessor connected to the main circuit board; at least one input deviceconnected to the main circuit board and to the processor through themain circuit board; and at least one output device connected to the maincircuit board and to the processor through the main circuit board. 2.The device of claim 1, wherein the first signal trace and dielectricsubstrate layer are integrally formed as part of a flexible printedcircuit board.
 3. The device of claim 2, wherein the flexible printedcircuit board comprises one or more stripline circuit layers overlayingthe dielectric substrate layer, each stripline circuit layer comprisinga pair of ground planes, a further dielectric substrate layer betweenthe ground planes, a further signal trace embedded within the furtherdielectric substrate layer, and multiple conductive vias extendingthrough the flexible printed circuit for connecting the ground planes ofeach circuit layer of flexible printed circuit board to the chassis;wherein all signal traces of the flexible printed circuit are connectedto the main circuit board.
 4. The device of claim 3, wherein theflexible printed circuit board further comprises at least one microstripcircuit layer overlaying the one or more stripline circuit layers, themicrostrip circuit layer comprising a second ground plane, a thirddielectric layer overlaying the second ground plane, a second signaltrace overlaying the third dielectric layer, and a fourth dielectriclayer overlaying the second signal trace.
 5. The device of claim 4,wherein the chassis is made from a conductive magnesium or stainlesssteel alloy and the fourth dielectric layer is air.
 6. The device ofclaim 1, wherein the chassis defines a recess, the main circuit boardbeing received in the recess, wherein the main circuit board is attachedto the chassis within the recess using a conductive adhesive.
 7. Thedevice of claim 1, further comprising an electronic component connectedto the first signal trace, wherein the main circuit board has a stack-upconfigured to control the impedance of the first signal trace to withina predetermined tolerance of a predetermined impedance threshold.
 8. Thedevice of claim 1, wherein the first and second dielectric substratelayers are integrally formed to form a compound dielectric substratelayer in which the first signal trace is embedded, the electricalassembly further comprising a group layer within the compound dielectricsubstrate layer forming a second ground plane, and wherein the chassisforms multiple conductive vias connecting the first ground plane to thesecond ground plane.
 9. The device of claim 2, wherein the chassisdefines a recess, the flexible printed circuit board being received inthe recess.
 10. The device of claim 9, wherein the recess includes afirst enlarged portion in a top portion of the chassis, a secondenlarged portion in a bottom portion of the chassis, and a generallyelongate portion which defines a path extending between the first largeportion in the top portion of the chassis and the second enlargedportion in the bottom portion of the chassis.
 11. The device of claim10, further comprising at least one first rigid printed circuit board inthe first enlarged portion in the top portion of the chassis and atleast one second rigid printed circuit board in the second enlargedportion in the bottom portion of the chassis, wherein the flexibleprinted circuit board connects the at least one first rigid printedcircuit board and the at least one second rigid printed circuit boards.12. The device of claim 1, wherein the chassis is made from a conductivemagnesium or stainless steel alloy.
 13. The device of claim 2, furthercomprising an electronic component connected to the first signal trace,wherein the flexible printed circuit board has a stack-up configured tocontrol the impedance of the first signal trace to within apredetermined tolerance of a predetermined impedance threshold.